This invention relates to "point" (very small) sources of radiation used for imaging applications, particularly to those point sources of ions and vacuum ultraviolet (VUV) light used for transfer of patterns during the fabrication of very small structures such as integrated circuits.
Point sources can be used for forming images of patterns by either of two processes. If the radiation can be focused by lenses, as can electrons, ions, or visible light, the radiation emitted by the source can be focused down into a small spot, which can be moved laterally across the surface of a target to write a pattern sequentially. If the radiation cannot be focused, as is the case with x-rays or VUV light, proximity printing, in which a mask is placed very close to the surface of a target, can be used to form an image of the mask pattern on the target. For proximity printing, it is necessary for the source to be as small as possible to prevent blurring of the image.
Because of the high energy densities that are usually present in point sources, the conditions are often not completely controllable, and more than one type of radiation is emitted from the source. This is the case, for example, with liquid metal ion sources, wherein visible light as well as ions are emitted from the tip of a cone formed by the liquid metal. It is conceivable that each type of radiation produced by a point source could be put to a practical use, although this is rarely done.
Point sources of ions are presently being used in machines being sold commercially for x-ray mask repair. In these machines, finely focused ion beams of ions formed from liquid metals such as gallium are used to observe and sputter etch the masks. The potential exists also to do lithography or implantation with these machines, if suitable ion sources can be developed. Lithography can be done with light ions such as H.sup.+, H.sub.2.sup.+, or He.sup.+, and implantation into gallium arsenide can be done with ions from the group H.sup.+, Zn.sup.+, Cd.sup.+, Si.sup.+, Sn.sup.+, and Be.sup.+, while implantation into silicon can be done with ions from the group B.sup.+, N.sup.+, Al.sup.+, Ga.sup.+, As.sup.+, In.sup.+, and Sb.sup.+.
Similarly, various point sources of x-rays have been under development for proximity printing, but success has not been immediate. The potential exists to use VUV radiation for proximity printing, since less flux is needed for exposure of lithographic resists. A discussion of VUV lithography is given by G. Coleman and R. A. McCorkle in their article "Ultra-soft x-ray lithography" in the IBM Technical Disclosure Bulletin, Vol. 24, No. 11B, P. 5804, April 1982. As with ion sources, however, common VUV sources lack sufficient brightness. A discharge constructed according to the present invention and utilizing either hydrogen or one of the noble gases provides the required brightness.
A general discussion of ion and light sources used in the microelectronics industry is given in The Physics of Microfabrication, by I. Brodie and J. Muray, Plenum, New York (1982).
The U.S. Pat. No. 4,549,082 describing how to make an ion source by charge exchange of an ion beam with a molecular beam has been issued to the present inventor.
The mechanism of microwave breakdown in gases has been discussed by S. Brown in The Encyclopedia of Physics, Vol. 22, p. 531 (1956).
A description of a microwave plasma disk ion source has been published by J. Asmussen and J. Root in Applied Physics Letters, Vol. 44, p. 396 (1984).
A description of a microwave discharge atomic hydrogen source has been published by E. Murphy and J. Brophy in The Review of Scientific Instruments, Vol. 50, p. 635 (1979).
A description of a microwave discharge VUV radiation source is given by E. Pellach and H. Sar-el in The Journal of Electron Spectroscopy and Related Phenomena, Vol. 14, p. 259 (1978).
Plasma jets formed from gases at atmospheric pressures using radiofrequency plasma generation, and used for welding torches, are discussed in Physics and Technology of Low Temperature Plasmas, ed. by S. Dresvin, Iowa (1977).
A discussion of sources used for x-ray lithography is given by A. Heuberger in Solid State Technology, February 1986, p. 93.
The background for laser radiation induced gas breakdown, including a discussion of cascade ionization, has been presented by D. Smith and R. Meyerrand, Jr. in Principles of Laser Plasmas, G. Bekefi, ed., Wiley, New York (1976), p. 457.
An ion source in which a molecular beam is transformed into an ion beam by stepwise excitation of states of the atoms in the beam using tuned lasers is described by R. Dreyfus and R. Hodgson in U.S Pat. No. 3,914,655.
An ion source wherein individual atoms in a rarefied gas are ionized by multiphoton ionization using a powerful pulsed laser, and where the gas density is not sufficiently high for cascade ionization, is described by W. Brubaker et al. in U.S. Pat. No. 3,478,204.